GB2235526A

GB2235526A - Optical pyrometer

Info

Publication number: GB2235526A
Application number: GB9017249A
Authority: GB
Inventors: Laurence Nicholas John
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1989-08-10
Filing date: 1990-08-07
Publication date: 1991-03-06
Anticipated expiration: 2010-08-07
Also published as: DE4024961A1; JPH03144326A; FR2650888A1; GB9017249D0; GB8918315D0; GB2235526B

Abstract

An optical pyrometer has a diffraction grating 3 which disperses radiation from a hot chamber 2 into a linear spectrum 31 of its component wavelengths. A vibrating beam 41 of a silicon microstructure 4 is excited by an optical source 44 to move along the spectrum and block passage of different wavelengths to a single photodiode 50 on which the radiation is focussed. A processor 51 receives the output of the photodiode 50 together with an output from the source 44 indicative of the position of the vibrating beam 41 along the spectrum and from this produces an output representative of the temperature of the chamber 2. <IMAGE>

Description

OPTICAL SYSTEMS This invention relates to optical systems The invention is

more particularly concerned with optical systems of the kind in which a broad spectrum of wavelengths of optical radiation is detected, the shape of the spectrum providing an indication of an input variable. For example, in optical pyrometry, the temperature of a hot body can be determined by monitoring the level of radiation emitted at different wavelengths.

The problem with such systems is that the accuracy with which the shape of the spectrum can be determined is 1 limited by the number of detectord employed but that increasing the number of detectors produces a corresponding increase in complexity, size, weight and cost of the system.

It is an object of the present invention to provide an optical system that can be used to avoid, at least in part, this problem.

2 According to one aspect of the present invention there is provided an optical system including means for dispersing radiation from a source of broad band optical radiation into its component wavelengths along a linear path so that different points along the path receive radiation of different wavelengths, a vibrating member located in the path, the vibrating member being arranged such that as it vibrates it moves along the path and selectively blocks transmission of radiation of different wavelengths, and detector means arranged to receive radiation passed by the vibrating member and a signal indicative of the position of the vibrating member on the path, and the detector means being arranged to provide an output representative of a wavelength characteristic of the source.

The vibrating member is preferably provided by a silicon microstructure and may include a beam supported at one end, the beam vibrating about its supported end. The system may include optical means arranged optically to drive the vibrating member to vibrate. The vibrating member may be opaque to optical radiation, the vibrating member being arranged to block transmission to the detector means of a small part of a different wavelengths. Alternatively, the vibrating member may be arranged to block transmission to the detector means of all but a small part of the different wavelengths. The detector means is preferably provided by a single photodiode on which is incident the radiation of all the wavelengths passed by the vibrating member. The means for dispersing radiation may be a diffraction grating. The system may be an optical pyrometer system and the source of broad band optical radiation may be provided by an optically emissive region, the system including processing means that is arranged to receive the output of the detector means and to provide an output representative of the temperature of the optically emissive region.

An optical pyrometer system in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows the system schematically; Figure 2 is a plan view of a part of the system; Figure 2A is a plan view of a part of a modified system; Figure 3 is a graph indicating the amplitude of radiation with wavelength supplied to the system; and Figure 4 is a graph indicating the variation of output amplitude with time of a part of the system.

With reference first to Figures 1 and 2, the pyrometer system includes a fibre-optic cable 1 by which optical radiation emitted from an optically emissive hot region or chamber 2 is supplied to a dispersing device 3. The dispersing device 3 disperses the radiation into a linear spectrum of its component wavelengths and supplies these via a vibrating member 4 to a detector unit 5 which determines the temperature of the chamber 2 from the shape of the radiation spectrum and provides an output to a display 7.

The cable 1 is of a conventional kind and has a high-temperature resistant lens 1.0 at its forward end which directs into the cable the broad band radiation emitted by the chamber 2, such as a gas-turbine engine combustion chamber. The radiation which emerges from the rear end of the cable 1 is focussed by a lens 11 into a beam which is directed onto the dispersion device 3 which takes the form of a transmission diffractive grating. Radiation of different wavelengths are diffracted at different angles from the grating 3 so that the radiation is dispersed laterally of the direction in which it is directed. The dispersed radiation is focussed by means of a lens 30 to form a linear spectrum image 31, as shown in Figure 2, which extends in the plane and transversely of the vibrating member 4. The spectrum image 31 varies in colour along its length such as from red at one end to blue at its opposite end. The intensity of radiation at different points along the image 31 will vary with temperature of the chamber 2 and the part of the spectrum utilised will depend on the response of the detector unit 5 employed.

The vibrating member 4 comprises a silicon microstructure which is etched or machined from a solid block of silicon and has a horizontal beam 41 supported at one end on an integral pillar 42. An optical drive fibre 43 is located to one side of the beam 41 and supplies pulses of radiation from a source 44 which are incident on the beam and cause it to vibrate at the frequency of the pulses. The vibrations of the beam 41 are oscillating annular movements about the supported end of the beam and occur in a horizontal plane which sweeps out a horizontal path through the dispersed beam of radiation from the grating 3. The beam 41-is opaque to optical radiation so that it acts as a mask that blocks passage of a small part of the different wavelengths of radiation. According to the position of the beam 41 in its vibration, it prevents passage of different wavelengths of radiation.

All the wavelengths of radiation which passes the vibrating microstructure 4 is incident on a single photodiode 50 in the detector unit 5. The aperture of the photodiode 50 is selected to receive incident on it the 1 cl entire part of the spectrum image 31 of interest, in this case that part including the colours red to blue. The output of the photodiode 50 is supplied to a processing unit 51 within the detector unit 5 which also receives an input from the drive source 44 in synchronism with the drive signal supplied to the drive fibre 43 which is representative of the position of the beam 41.

The output from the photodiode 50 will vary as the beam 41 sweeps along the spectrum image 31, unless the radiation spectrum from the chamber 1 is completely flat within the spectrum image. Figure 3 shows an example of a broad band spectrum of radiation that might be emitted. from the cavity 2. The range of wavelengths masked by the beam is indicated byg. It can be seen that when the beam is located at one extreme of its travel along the spectrum image 31, at the shortest wavelengths, only the wavelengthsgXl, will be obscured. Because the amplitude of radiation is low in this region the effect on the overall level of radiation passed to the photodiode 50 will be small and hence the output of the photodiode, as shown in Figure 4, will be higher. As the beam 41 moves to obscure slightly longer wavelengthsg t, it will be located in a region of intense radiation and will, therefore, have a greater effect on the overall level of radiation, photodiode leading to a reduction in the output of the 50. This is illustrated by the trough in the graph of Figure 4. The output of the photodiode 50 in time will, therefore, be inverse to the amplitude/wavelength spectrum of radiation emitted by the chamber 2. As the beam 41 moves back in the opposite direction, the output of the photodiode 50 will vary in the opposite sense. The processing unit 51 carries out suitable processing on the signal output from the photodiode 50 which may include storage and averaging over several periods of vibration of the beam, as signalled by the output from the drive source 44 supplied to the processing unit 51. The processing unit 51 may blank out the output.of the.photodiode 50 on alternate scans so that only scans in the same direction are utilised. In the present pyrometry system, the processing typically includes inversion and scaling of the output of the photodiode so that the processed signal is directly representative of the amplitude of radiation emitted by the chamber 1. This signal is then sampled at two different times T. and T,, during each period so as to to obtain an indication of the intensity of radiation at two different wavelengths _ A andi,,. By comparing these intensities an indication of the temperature of the cavity can be provided in a way that is common practice in pyrometry.

1 The present invention enables the shape of a spectrum to be indicated using only one detector. This avoids the problem of previous systems employing several detectors where the response of the detectors may change relative to one another in time.

It will be appreciated that the invention is not confined to pyrometry applications but can be used in other applications where an indication of the wavelength characteristic of a broad band source, such as its spectrum shape, needs to be measured.

The invention could be modified so that the vibrating member prevents all but a small band of wavelengths S'X being passed to the detector. For example, as shown in Figure 2A, the vibrating member could be a mask 60 with a slit 61 that is moved along the path over which radiation is dispersed so that it blocks transmission to the detector of all but a small part of the different wavelengths. Alternatively, the vibrating member could be reflective so that it reflects to the detector a changing small band of wavelengths.

The means by which the radiation is dispersed into its component wavelengths need not be a transmission diffraction grating but could, for example, be a reflective diffractive grating or a prism.

The vibrating member need not he optically driven, as described above, but could be driven, for example, electrostatically or piezoelectrically. The vibrating member could be an electromagnetic or other device and need not be a silicon microstructure although this does enable a very compact construction to be achieved.

Claims

An optical system including means for dispersing radiation from a source of broad band optical radiation into its component wavelengths along a linear path so that different points along the path receive radiation of different wavelengths, vibrating member located in said path, said vibrating member being arranged such that as it vibrates it moves along said path and selectively blocks transmission of radiation of different wavelengths, and detector means arranged to receive radia±ion passed by the vibrating member and a signal indicative of the position of said vibrating member on said path, and wherein the detector means is arranged to provide an output representative of a wavelength characteristic of the source.

An optical system according to Claim 1, wherein the vibrating member includes a silicon microstructure.

An optical system according to Claim 1 or 2, wherein the vibrating member includes a beam supported at one end, and wherein the beam vibrates about its supported end.

An optical system according to any one of the preceding claims including optical means arranged optically to drive the vibrating member to vibrate.

5.

An optical system according to any one of the preceding claims, wherein the vibrating member is opaque to optical radiation, and wherein the vibrating member is arranged to block transmission to the detector means of a small part of the different wavelengths.

An optical system according to any one of Claims 1 to 4, wherein the vibrating member is opaque to optical radiation, and wherein the vibrating member is arranged to block transmission to the detector means of all but a small part of the different wavelengths.

I i An optical system according to any one of the preceding claims, wherein the detector means is provided by a single photodiode on which is incident the radiation of all the wavelengths passed by the vibrating member.

An optical system according to any one of the preceding claims, wherein the means for dispersing radiation includes a diffraction grating.

An optical system substantially as hereinbefore described with reference to Figures 1, 2, 3 and 4 of the accompanying drawings.

10.

11.

An optical system substantially as hereinbefore described with reference to Figures 1, 2, 3 and 4 as modified by Figure 2A of the accompanying drawings.

An optical pyrometer system including an optical system according to any one of the preceding claims, wherein the said source of broad band optical radiation is provided by an optically emissive region, and wherein the system includes processing means arranged to receive the output of said detector means and to provide an output representative of the temperature of said optically emissive region.

14 - 12.

An optical pyrometer system substantially as hereinbefore described with reference to Figures 1, 2, 3 and 4 of the accompanying drawings.

1 W 13.

An optical pyrometer system substantially as hereinbefore described with reference to Figures 1, 2, 3 and 4 as modified by Figure 2A of the accompanying drawings.

14.

Any novel feature or combination of features as hereinbefore described.

Published 1991 at The Patent Office. State House. 66/71 High Holborn. LondonWC I R47P. Further copies rnay be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwrnrclinfach. Cross Keys. Newport. NP1 7HZ- Printed by Multiplex techniques ltd. St Mary Cray, Kent.